1. Field of the Invention
The present invention relates to a substrate holding apparatus for holding a substrate to be polished and pressing the substrate against a polishing surface, and more particularly to a substrate holding apparatus for holding a substrate such as a semiconductor wafer in a polishing apparatus for polishing the substrate to a flat finish. The present invention also relates to a polishing apparatus having such a substrate holding apparatus.
2. Description of the Related Art
In recent years, semiconductor devices have become smaller in size and structures of semiconductor elements have become more complicated. In addition, the number of layers in multilayer interconnects used for a logical system has been increased. Accordingly, irregularities on a surface of a semiconductor device become increased, and hence step heights on the surface of the semiconductor device tend to be larger. This is because, in a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, then micromachining processes, such as patterning or forming holes, are performed on the semiconductor device, and these processes are repeated many times to form subsequent thin films on the semiconductor device.
When the number of irregularities on a surface of a semiconductor device is increased, a thickness of a thin film formed on a portion having a step tends to be small. Further, an open circuit is caused by disconnection of interconnects, or a short circuit is caused by insufficient insulation between interconnect layers. As a result, good products cannot be obtained, and the yield tends to be reduced. Furthermore, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after a long-term use. At the time of exposure in a lithography process, if a surface to be irradiated has irregularities, then a lens unit in an exposure system cannot focus on such irregularities. Therefore, if the irregularities of the surface of the semiconductor device are increased, then it becomes difficult to form a fine pattern on the semiconductor device.
Accordingly, in a manufacturing process of a semiconductor device, it becomes increasingly important to planarize a surface of a semiconductor device. The most important one of the planarizing technologies is CMP (Chemical Mechanical Polishing). Chemical mechanical polishing is performed with use of a polishing apparatus. Specifically, a substrate such as a semiconductor wafer is brought into sliding contact with a polishing surface such as a polishing pad while a polishing liquid containing abrasive particles such as silica (SiO2) is supplied onto the polishing surface, so that the substrate is polished.
This type of polishing apparatus comprises a polishing table having a polishing surface constituted by a polishing pad, and a substrate holding apparatus, called a top ring or a carrier head, for holding a semiconductor wafer. A semiconductor wafer is polished by the polishing apparatus as follows: The semiconductor wafer is held by the substrate holding apparatus and then pressed against the polishing table under a predetermined pressure. At this time, the polishing table and the substrate holding apparatus are moved relative to each other for thereby bringing the semiconductor wafer into sliding contact with the polishing surface. Accordingly, the surface of the semiconductor wafer is polished to a flat mirror finish.
In such a polishing apparatus, if a relative pressing force between the semiconductor wafer being polished and the polishing surface of the polishing pad is not uniform over an entire surface of the semiconductor wafer, then the semiconductor wafer may be insufficiently be polished or may excessively be polished at some portions depending on the pressing force applied to those portions of the semiconductor wafer. In order to avoid such a drawback, it has been attempted to form a surface, for holding a semiconductor wafer, of a substrate holding apparatus with use of an elastic membrane (membrane) made of an elastic material such as rubber and apply a fluid pressure such as an air pressure to a backside surface of the elastic membrane so as to uniform a pressing force applied to the semiconductor wafer over an entire surface of the semiconductor wafer. In this case, the elastic membrane is made of ethylene propylene rubber (EPDM), polyurethane rubber, fluororubber, or the like.
As described above, in the so-called floating-type substrate holding apparatus in which a holding surface for holding a semiconductor wafer is formed by an elastic membrane, since the semiconductor wafer is pressed by the elastic membrane, the elastic membrane and the semiconductor wafer are brought into close contact with each other. Thus, after polishing of the semiconductor wafer is finished, when the semiconductor wafer is removed from the substrate holding apparatus, the semiconductor wafer is difficult to be detached from the elastic membrane. In this case, if the semiconductor wafer is forced to be detached from the elastic membrane, then the semiconductor wafer is possibly damaged or broken. Further, the hardness of the elastic membrane is made low so that the elastic membrane can press the semiconductor wafer up to the outer peripheral portion of the semiconductor wafer, and hence the degree of adhesion between the elastic membrane and the semiconductor wafer is further increased. Thus, the semiconductor wafer is more difficult to be detached from the elastic membrane.